OK, I see. Your original post said a 50% loss with the cover on. 20% seems right to me. After a few days of 80 degree temps. up here that prompted me to open the pool (and the kids to swim despite water temps. in the mid. 60's), it has been around 60 during the day and upper 30's at night (with one night of 28, yikes!). I run my heat pump from 9am to 9pm as there is no point in running it at night with those temps). I only bother to run it at all as it does take a few days to warm the water up to swimming temp. and since I already have it there I want to be ready for our next warm day.

So I would say that I have it raised 10 to 15 degrees over where it would be and lose 2 or 3 degrees of that each night with the cover on and the pump off, so the 20% is right on the money.